The present invention relates to an improved process for etherifying celluloses. More particularly, it relates to a process for preparing cellulose ethers of a high quality in which the substituents introduced thereinto by etherification are uniformly distributed.
Heretofore, there has been known a process for preparing cellulose ethers in which celluloses such as unsubstituted cellulose, methyl cellulose, hydroxyethyl cellulose and carboxymethyl cellulose are treated with alkali hydroxide to give an alkali cellulose, and the alkali cellulose is reacted with an etherifying agent such as alkyl halide. It has also been known that the quality of the cellulose ether prepared according to such a process varies widely depending upon the uniformity of the distribution of the substituents introduced thereinto by etherification. That is, a cellulose ether in which the substituents are uniformly distributed is of a high quality, i.e. good in solubility to solvents and film-forming property in coating. However, a cellulose ether in which the substituents are not uniformly distributed is of a low quality, i.e. poor in solubility to solvents and film-forming property in coating.
In preparing cellulose ethers according to the prior process, an etherifying agent such as alkyl halide is consumed by side reaction with water being present in the reaction system as well as by reaction with the alkali cellulose. Generally, for this reason, a highly concentrated aqueous solution of alkali hydroxide is employed in preparing the alkali cellulose in order to utilize the etherifying agent effectively. However, the employment of such a highly concentrated aqueous solution of alkali hydroxide causes another disadvantage that alkali hydroxide is not uniformly dispersed into the celluloses, so that a cellulose ether in which the distribution of the substituents introduced thereinto by etherification is non-uniform tends to be formed. Furthermore, the prior process has a fatal defect that the etherification reaction is a heterogeneous reaction which is carried out in a heterogeneous system consisting essentially of an aqueous phase, an organic liquid phase (etherifying agent) and a solid phase (alkali cellulose), so that a cellulose ether in which the distribution of the substituents is non-uniform tends to be formed.
Attempts for preparing cellulose ethers in which the distribution of substituents is uniform have been proposed including a process in which an alkali cellulose of a uniform quality is prepared and it is then subjected to etherification, as described in Japanese Patent Publication No. 12954/1978 and a process in which a solid alkali hydroxide is added in two or more portions during etherification reaction, as described in U.S. Pat. No. 2,254,249. However, these processes have a disadvantage that the reaction procedures are troublesome, and the abovementioned disadvantage due to the hetergeneous reaction is not fundamentally settled.
Recently there has been an attempt for settling the disadvantage due to the heterogeneous reaction in which the etherification reaction is carried out in an aprotic polar solvent such as dimethyl sulfoxide. However, the aprotic polar solvent is expensive, and it is difficult to recover the solvent from the reaction mixture. Furthermore, it is difficult to raise the feed concentration of celluloses since the solubility of celluloses in the aprotic polar solvent is small. For these reasons, the process is disadvantageous for industrial purpose.
On the other hand, the utility of cellulose ethers varies depending upon the viscosity of their solution, in other words, their molecular weight. Even though in the past cellulose ethers giving a solution of a medium viscosity were mainly employed, recently cellulose ethers giving a solution of a high or low viscosity have become important. In particular, the demand for cellulose ethers giving a solution of a low viscosity (hereinafter referred to as "low viscosity cellulose ether") has increased in the field of coatings, and there has been required the development of an economical process for preparing low viscosity cellulose ethers of a high quality.
The preparation of a low viscosity cellulose ether is carried out according to the following two processes: One is a process in which preformed celluloses having a low molecular weight are subjected to etherification. The other is a process in which a preformed cellulose ether is subjected to depolymerization until its molecular weight is lowered to give a desired low viscosity cellulose ether.
The former process includes a process for preparing a low viscosity mixed cellulose ether containing carboxymethyl group from carboxymethyl cellulose (CMC) having a low molecular weight. Generally a mixed cellolose ether containing carboxymethyl group is prepared by reacting a mercerized CMC with an etherifying agent such as alkyl halide. The quality of the resulting mixed cellulose ether is markedly influenced by the uniformity of the mercerized CMC as a starting material. The conditions required for the mercerized CMC are as follows: The distribution of alkali hydroxide therein is uniform. The mercerized CMC contains a sufficient and minimum amount of alkali hydroxide for the reaction with an etherifying agent and a minimum amount of water unless the uniformity of the mercerized CMC is impaired in order to prevent disadvantageous phenomena such as side reaction.
In order to obtain a mercerized CMC satisfying the foregoing conditions, it is assumed that the immersion of CMC in a highly concentrated aqueous solution of alkali hydroxide at an elevated temperature for a short time is preferable. In fact, however, it is difficult to obtain a uniform mercerized CMC according to the mercerization process or to practice the mercerization process due to the restriction caused by the physical properties of CMC as a starting material. For this reason, practically there has been adopted a mercerization process in which CMC is immersed in a low concentrated aqueous solution of alkali hydroxide at a low temperature for a long time and thereafter the resultant is squeezed. The process is hereinafter referred to as "immersing and squeezing process". However, the immersing and squeezing process requires a long production time and much labor. It is difficult to control the concentrations of alkali hydroxide and water in the resulting mercerized CMC. Moreover, the mercerized CMC prepared according to the process should be ground before it is subjected to etherification. Thus the immersing and squeezing process has various problems in the production efficency and the process control.
Furthermore, when a low viscosity mercerized CMC, i.e. a mercerized CMC having a low molecular weight is prepared according to the immersing and squeezing process, an aging for a much long period of time or under drastic reaction conditions is required for cleaving the main chain of CMC. However, CMC tends to be undesirably discolored since it is subjected to the strong action of alkali hydroxide during the aging. Moreover, since the cleavage of the main chain of CMC is carried out in a heterogeneous system, large amounts of components having an extremely short chain length tend to be formed. When a mixed cellulose ether is prepared by employing a mercerized CMC containing such components as a starting material, an etherified CMC corresponding to the components should be removed from the resulting mixed cellulose ether by purification, which results in the low yield of a desired mixed cellulose ether and inevitably the increase in cost, and the increase in the chemical oxygen demand of the waste water exhausted from this step.
It is well known that generally the autoxidation of alkali cellulose is accelerated by a transition element such as cobalt, manganese or iron. In case of mercerizing CMC to prepare a low viscosity mercerized CMC, it is possible to reduce somewhat the aging time by adding such a transition element. However, the catalytic effect of the transition element is not so great. A mixed cellulose ether prepared by employing a mercerized CMC obtained in such a manner is inevitably contaminated by the transition element. Therefore, the aging employing the transition element is undesirable for preparing a mixed cellulose ether employed for use in which the contamination with the transition element should be avoided.
The prior arts relating to the above-mentioned latter process for preparing a low viscosity cellulose ether, particularly a low viscosity carboxymethyl ethyl cellulose, in which a preformed cellulose ether is subjected to depolymerization are as follows: Many processes have been proposed for preparing low viscosity cellulose ethers but generally a depolymerization process in which the main chain of cellulose ether is cleaved by oxidation or acid is adopted.
The depolymerization process employing acid has disadvantages such as the requirement of troublesome procedures and a neutralizing agent for removing the acid employed as a catalyst after the completion of reaction, and the requirement of an acid-resistant reaction vessel. In case of a cellulose ether employed as a coating agent, the presence of a slight amount of an acid or inorganic salt is undesirable since the acid or inorganic salt tends to make the cellulose ether insoluble and to cause the discoloration of the cellulose ether, which are fatal defects as coating agent. However, it is known that sulfuric acid among common acids is readily bonded with celluloses and the removal of the bonded sulfuric acid is difficult. For this reason, there is usually adopted, as a depolymerization process employing acid, a process in which a dried powdered cellulose ether is reacted with a hydrogen halide gas such as hydrogen chloride gas according to fluidized bed process or a process in which a powdered cellulose ether in the form of a slurry in an inert organic solvent is reacted with a hydrogen halide gas in a mixing device such as rotary mixer. As to the depolymerization process in the slurry, it is known that the organic solvent employed should be inert to the starting cellulose ether and substantially anhydrous and that the presence of water or alcohol is undesirable when it is desired that the initial degree of substituent in the cellulose ether is maintained after depolymerization. However, even in the depolymerization process employing such a volatile acid, a neutralizing agent is required to remove a trace amount of the acid. Furthermore, the depolymerization process has a fatal defect that it is difficult to obtain a low viscosity cellulose ether of a high quality. That is, the low viscosity cellulose ether prepared according to the process is discolored to yellow or brown. The degree of depolymerization is non-uniform due to the depolymerization in the substantially heterogenous system. It is difficult to remove impurities from the resulting cellulose ether.
In the depolymerization process employing oxidation reaction, the oxidation is carried out by employing air or peroxide. The oxidation by air has a defect that the rate of depolymerization is small and the oxidation by peroxide is generally adopted. According to the oxidation by peroxide, it is difficult to remove the peroxide remaining in final product and side reactions tend to occur so that the degree of substitution in final product is lowered. An attempt for settling such defects is proposed in which a high viscosity cellulose ether is mixed with an aqueous solution of hydrogen peroxide and the mixture is dried at 100.degree. to 250.degree. C. until hydrogen peroxide is consumed completely or nearly completely (see Japanese Patent Publication No. 678/1970). However, the drying at an elevated temperature of 100.degree. to 250.degree. C. causes an undesirable heat deterioration which results in the discoloration and insolubilization of the final product. In order to prepare a cellulose ether for use in which such discoloration and insolubilization are undesirable, the purification of final product by which the final product is bleached and insoluble materials are removed is further required.
Such a purification process is known with respect to high viscosity cellulose ethers. That is, U.S. Pat. No. 3,549,617 describes a process in which a high viscosity cellulose ether is slurried in an aqueous solution of a lower alcohol containing sulfite ion and agitated. The purification process described in the U.S. patent is effective for a high viscosity cellulose ether which is insoluble in the aqueous solution of a lower alcohol but ineffective for a low viscosity cellulose ether, for instance, prepared by depolymerization employing hydrogen chloride since the low viscosity cellulose ether is gelled by the water in the aqueous solution or is dissolved into the aqueous solution.
As described above, the conventional depolymerization process and purification process for cellulose ethers have various problems. Therefore, when the conventional processes are applied to the depolymerization and purification of carboxymethyl ethyl cellulose being one of cellulose ethers, the same problems are encountered.
It is known that carboxymethyl ethyl cellulose is suitable as a coating agent for enteric medicaments (see U.S. Pat. Nos. 3,789,117 and 3,896,108). However, carboxymethyl ethyl cellulose employed as a coating agent for enteric mdeicaments is required to give a solution having a proper viscosity relating to its coating property and solubility to intestinal juice from view-point of practical use and to have a superior whiteness in the form of solid and a superior transparency in the form of solution from viewpoint of commercial value. There is a great demand for preparing carboxymethyl ethyl cellulose satisfying such requirements.